1. Field of the Invention
This invention relates to methods of optimizing laser amplifier output. In particular, this invention relates to optimizing laser amplifier output in ultrashort-pulse multipass laser amplifiers, by selective use of a spectral filter.
2. Description of Related Art
Gain narrowing has long been recognized as a problem in amplifiers where the beam passes through the amplifying medium multiple times (e.g. multipass configurations and regenerative configurations). Both multipass and regenerative amplifiers pass the beam being amplified through the gain material a number of times, in order to achieve sufficient amplification. Since gain follows a generally bell-shaped curve, the repeated passes result not just in increased power, but also in narrowed gain profile.
Gain-flattening etalons and other types of filters have been used in the past in room temperature regenerative [4] and multipass amplifiers [3, 5]. The resulting gain shape achieved is not ideal however, as it is not flat but rather a double peak shape or “hard edges” on the spectrum. See, for example, FIG. 4 in reference [6], wherein the double peak has completely suppressed the gain at 800 nm, the center frequency of the input pulse.
This non-ideal effect of the etalon or filter is minor in early passes, because the gain is high and offsets the “double peak” effect of the filter. In later passes, when amplifier is near saturation, the double peak effect predominates. Furthermore, the amount of gain the amplifier can achieve is degraded by use of a filter in these last few passes when the power is the highest. Such filters work best in small signal regimes. However, no prior work in the field discusses the insertion of such a filter in earlier passes but not later passes, although it is the case that some laser systems have used two stages of amplification—the first of which did include the filter, and the second of which did not and others [7, 8, 9] have removed the pulse from the amplifier before a final pass through the filter. In others, the position of the filter was such that the pulse was removed from the amplifier after passing through the crystal, but before a final pass through the filter. However, no attempt was made in past work to fabricate and mount the filter in such a way that the number of passes through which the filter is inserted can be varied.
A need remains in the art for a technique of optimizing the output of ultrashort-pulse multipass amplifiers by selectively inserting a spectral filter in the path of the beam being amplified.